1. Field of the Invention
The present invention relates to a method of determining the position of a catheter in a patient's esophagus, a control unit for use with a ventilator, and a computer program product for use in such a control unit.
2. Description of the Prior Art and Related Applications
It is known in the art to use myoelectrical or neuroelectrical signals from a patient to control the function of a ventilator providing breathing support to the patient. U.S. Pat. No. 5,820,560 and U.S. Pat. No. 6,588,423 both disclose methods and devices for triggering ventilatory support to a patient using a myoelectrical signal obtained from the diaphragm. U.S. Pat. No. 5,671,752 discloses a method and a device for registering the myoelectrical activity of the diaphragm by means of an esophageal catheter having an array of electrodes. The signals from such a catheter can be used as the myoelectrical signal to control ventilator function. EP 1 091 780 discloses the use of a neuroelectrical signal picked up, for example, from the phrenic nerve to control a ventilator.
A problem when obtaining a myoelectrical signal from the diaphragm is positioning of the catheter within the patient's esophagus. To obtain a proper signal some of the electrodes of the catheter should be placed above the diaphragm and some below it. There is a possibility that the catheter will be inserted too far, or not be inserted far enough. In both cases, the catheter will detect a weak signal, or may not capture any signal at all. Alternatively, the catheter may capture myoelectrical signals from other muscles instead of, or in addition to the signal from the diaphragm. Hence, it is difficult to obtain an optimal catheter position and the ventilator may have to work in pneumatic triggering mode if the signal is too weak.
There are some problems associated with methods based on the registration of the EMG signal from the diaphragm.                There may not be an EMG signal present, for example if the patient is sedated or has no breathing activity of his own for other reasons.        The EMG signal may be very weak and/or difficult to detect, for example because of disturbances caused by breathing support provided to the patient.        There is a risk that other myoelectric signals resembling that of the diaphragm but originating from other muscles are mistaken for signals from the diaphragm. Such signals may come, for example, from the abdominal muscles.        
Co-pending application No PCT/EP2007/054149 discloses a method of positioning the catheter based on the ECG component that will always be present in a myoelectrical signal from the diaphragm. In this application, the damping of the ECG signal caused by the diaphragm is used. The ECG signal components from different electrode pairs are determined and compared and the difference in amplitude of the ECG signal between different electrode pairs is used to determine the position of the diaphragm relative to the electrode pairs. The greatest damping between two neighbouring electrode pairs should be caused by the diaphragm being positioned between these two electrode pairs. This method is predominantly based on the registration and comparison of the QRS complex of the ECG signal.
Co-pending Swedish patent application No. 0850076-1 discloses a method utilizing the presence of the p wave of the ECG signal. Since the damping of the p wave is very strong with increasing distance from the heart, any electrode pair picking up a p wave must be located fairly close to the heart's atria, well above the diaphragm.